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HYDROSULPHITES

69

sulphuric acid and titrated in the cold with permanganate. With a solution
of 5-65 grams of pure potassium permanganate per litre, each c.c. corresponds
with i vol. of oxygen ; using N/io-permanganate, I c.c. = 0-5596 vol
of oxygen and I vol. of oxygen = 0-3 gram H2O2.

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* *

Commercial hydrogen peroxide usually contains 3-3-6% of H2O2, and thus
yields 10-12 vols. of oxygen. Nowadays solutions of 30% or 50% or even
higher concentrations are prepared, these giving 100, 160 or more vols, of oxygen.
That intended for bleaching textiles should contain only traces of iron, alumina
and barium1 ; an acidity corresponding with 1-5 gram of sulphuric acid per
litre (3 c.c. of N/io -alkali per c.c.) is allowable.2 Pure hydrogen peroxide for
pharmaceutical purposes should be neutral or neutralisable by 2 or 3 drops of
baryta and on evaporation should leave not more than 0-5 gram of fixed residue
per litre ; it should yield not less than 12 vols. of oxygen (Italian Pharmacopoeia).

HYDROSULPHITES

These are products used in dyeing and for decolorising sugar syrups,
etc., and have various compositions and names, such as Hydro sulphite,
Rongalite, Hyraldite, Decroline and Blankite. The active principle of these
products is sodium hydro sulphite, Na2S204, or sodium hydrosulphite-for-
maldehyde or sulphoxylate, NaHS02, CH2O + 2H20, or the corresponding
zinc salts. The action is regulated, according to circumstances, by addition
of zinc oxide, lithopone, small quantities of catalytic substances (indulinc
scarlet, patent blue, etc.). As impurities it may contain especially excess
of water, bisulphite, sulphite and sulphate. The value depends on the
content in active principle (hydrosulphite or sulphoxylate), which is deter-
mined as follows :

Quantitative Determination.—This is based on the reducing power
of the hydrosulphites on indigo : i part of sodium hydrosulphite corresponds
with 1-505 of pure indigo and i part of sodium sulphoxylate with 1-705
of indigo. The procedure is as follows :

(a) WITH SODIUM HYDROSULPHITE. 1*505 gram of purest indigo and
9 grains of sulphuric acid (monohydrate) are heated for 6 hours at 40-50°
with occasional shaking ; after cooling, the mass is poured into watei, the
solution filtered, the filter thoroughly washed, and the nitrate made up
to i litre.

Next 5 grams of the sample of hydrosulphite are dissolved in 10 c.c.
of sodium hydroxide solution (38-40° Baume), the volume being then made
up to 500 c.c. with recently boiled water and the air in the neck of the flask
replaced by illuminating gas. The solution is then drawn (not poured)
into a burette.

100 c.c. of the indigo solution are introduced into a conical flask
with a three-holed stopper traversed by the end of the burette con-
taining the hydrosulphite solution, by a gas entry tube reaching almost
to the surface of the liquid and by an exit tube. A current of illuminating

1  Sisley, " Sur 1'anal. de 1'eau oxyg." in Rev. g&n. des mat. colorantes, 1901, p, 209,
and 1904,  p. 164.

2  Sisley, loc. cit.rude calcium citrate contains 64-70% of crystallised citric acid (+ iH2O)